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multistage amp is used for the sake of larger gain.....
the maximum gain for a single stage cannot be correctly said.... it depends on the biasing conditions, amplifier config etc etc....
Not only for larger gain, but also when there are multiple requirements that you can't meet with a single stage. For example, if you need to drive a low impedance load, but still need fairly high gain, you can use the first stage to get the voltage gain and add a second stage to drive the load.
Hi there,
In a Power Amplifier, to optimize the PAE, the last stage operates close to saturation for linear modulations (like UMTS/WCDMA), and in deep saturation for constant envelope modulations (like GSM/GMSK). This means that the last stage does not have much gain. So you need multistage for the overall PA Gain, and to provide enough input power for the last stage to operate in optimum PAE condition.
To give you an example, most PAs in GSM applications use 3 stages, with input power of about 0dBm, and output power of about 34dBm.
As mentioned by psmon, Steve Cripps's books are very good for this subject.
Mosieur_Oiso
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